The maintenance of extremely low humidity environments is critical to a number of industries including: electronics manufacturing, (i.e. semi-conductors), pharmaceuticals, munitions storage, and specialty packaging. For example, the semi-conductor industry ships some of its products in extremely low humidity packaging (less than 5% relative humidity) to prevent corrosion. Even small amounts of humidity can corrode a semi-conductor, effect the quality of pharmaceuticals or destabilize munitions. A simple low-cost means for determining humidity levels is needed to monitor the integrity of such low humidity environments.
Humidity indicators range from simple visible indicators to complex electronic humidity devices. The most popular visible indicators employ cobalt salts (i.e. cobalt chloride) that change color when exposed to a certain level of relative humidity. See, for example U.S. Pat. Nos. 2,460,064 and 2,460,065 issued to Paul Bell Davis on Jan. 25, 1949. Cobalt salt indicators work well in moderate humidity levels but they are poor at indicating extremely low relative humidity levels (i.e. below 5%) at common ambient temperatures. Furthermore, in 2000 the European Union declared cobalt chloride, the most widely used humidity indicator salt, a class II carcinogen. This makes the use of cobalt chloride-based indicators undesirable.
Other types of visible indicators include devices that employ deliquescent chemicals or salts in combination with water soluble dyes. See, U.S. Pat. No. 2,249,867 issued to Snelling. Deliquescent indicators work in some situations, but their low heat resistance and tendency to undergo irreversible chemical change limit their potential use. Electronic humidity indicators have become more prevalent in recent years but they are not cost-effective for high volume applications.
In view of the prior art there is a need for a non-carcinogenic, cost-effective visual humidity indicator which can detect the presence of extremely low amounts of water vapor.